It is shown that (-)erythro-fluoro-citrate irreversibly binds to a site of the inner mitochondrial membrane of liver, brain and kidney mitochondria. At a concentration of 10 p moles/mg protein citrate translocation is specifically and completely inhibited. These results show that a citrate carrier has been identified by fluorocitrate, which serves as a probe for this membrane protein. Novel techniques for the measurement of anion transfer and a kinetic model for bidirectional ion flux was also developed for isolated mitochondria.